Dumbbell



FIG. 1 is a perspective view of our single design concept, showing our new design;

FIG. 2 is a top plan view of our single design concept, the bottom plan, front elevational view and back elevational view being identical to the view thereof;

FIG. 3 is an perspective view of an alternative embodiment of our single design concept, showing our new design;

FIG. 4 is a top plan view of our single design concept, the bottom plan, front elevational view and back elevational view being identical to the view thereof;.

FIG. 5 is an end view of FIG. 1; and,

FIG. 6 is an end view of FIG. 3. 

The ornamental design for a dumbbell, as shown and described. 